ext4: handle casefolding with encryption

This adds support for encryption with casefolding.

Since the name on disk is case preserving, and also encrypted, we can no
longer just recompute the hash on the fly. Additionally, to avoid
leaking extra information from the hash of the unencrypted name, we use
siphash via an fscrypt v2 policy.

The hash is stored at the end of the directory entry for all entries
inside of an encrypted and casefolded directory apart from those that
deal with '.' and '..'. This way, the change is backwards compatible
with existing ext4 filesystems.

[ Changed to advertise this feature via the file:
  /sys/fs/ext4/features/encrypted_casefold -- TYT ]

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20210319073414.1381041-2-drosen@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 28 insertions, 9 deletions

diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 110688d6b7a6..ffb295aa891c 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -55,6 +55,18 @@ static int is_dx_dir(struct inode *inode)
 	return 0;
 }
 
+static bool is_fake_dir_entry(struct ext4_dir_entry_2 *de)
+{
+	/* Check if . or .. , or skip if namelen is 0 */
+	if ((de->name_len > 0) && (de->name_len <= 2) && (de->name[0] == '.') &&
+	    (de->name[1] == '.' || de->name[1] == '\0'))
+		return true;
+	/* Check if this is a csum entry */
+	if (de->file_type == EXT4_FT_DIR_CSUM)
+		return true;
+	return false;
+}
+
 /*
  * Return 0 if the directory entry is OK, and 1 if there is a problem
  *
@@ -73,16 +85,20 @@ int __ext4_check_dir_entry(const char *function, unsigned int line,
 	const int rlen = ext4_rec_len_from_disk(de->rec_len,
 						dir->i_sb->s_blocksize);
 	const int next_offset = ((char *) de - buf) + rlen;
+	bool fake = is_fake_dir_entry(de);
+	bool has_csum = ext4_has_metadata_csum(dir->i_sb);
 
-	if (unlikely(rlen < EXT4_DIR_REC_LEN(1)))
+	if (unlikely(rlen < ext4_dir_rec_len(1, fake ? NULL : dir)))
 		error_msg = "rec_len is smaller than minimal";
 	else if (unlikely(rlen % 4 != 0))
 		error_msg = "rec_len % 4 != 0";
-	else if (unlikely(rlen < EXT4_DIR_REC_LEN(de->name_len)))
+	else if (unlikely(rlen < ext4_dir_rec_len(de->name_len,
+							fake ? NULL : dir)))
 		error_msg = "rec_len is too small for name_len";
 	else if (unlikely(next_offset > size))
 		error_msg = "directory entry overrun";
-	else if (unlikely(next_offset > size - EXT4_DIR_REC_LEN(1) &&
+	else if (unlikely(next_offset > size - ext4_dir_rec_len(1,
+						  has_csum ? NULL : dir) &&
 			  next_offset != size))
 		error_msg = "directory entry too close to block end";
 	else if (unlikely(le32_to_cpu(de->inode) >
@@ -94,15 +110,15 @@ int __ext4_check_dir_entry(const char *function, unsigned int line,
 	if (filp)
 		ext4_error_file(filp, function, line, bh->b_blocknr,
 				"bad entry in directory: %s - offset=%u, "
-				"inode=%u, rec_len=%d, name_len=%d, size=%d",
+				"inode=%u, rec_len=%d, size=%d fake=%d",
 				error_msg, offset, le32_to_cpu(de->inode),
-				rlen, de->name_len, size);
+				rlen, size, fake);
 	else
 		ext4_error_inode(dir, function, line, bh->b_blocknr,
 				"bad entry in directory: %s - offset=%u, "
-				"inode=%u, rec_len=%d, name_len=%d, size=%d",
+				"inode=%u, rec_len=%d, size=%d fake=%d",
 				 error_msg, offset, le32_to_cpu(de->inode),
-				 rlen, de->name_len, size);
+				 rlen, size, fake);
 
 	return 1;
 }
@@ -224,7 +240,8 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 				 * failure will be detected in the
 				 * dirent test below. */
 				if (ext4_rec_len_from_disk(de->rec_len,
-					sb->s_blocksize) < EXT4_DIR_REC_LEN(1))
+					sb->s_blocksize) < ext4_dir_rec_len(1,
+									inode))
 					break;
 				i += ext4_rec_len_from_disk(de->rec_len,
 							    sb->s_blocksize);
@@ -265,7 +282,9 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 
 					/* Directory is encrypted */
 					err = fscrypt_fname_disk_to_usr(inode,
-						0, 0, &de_name, &fstr);
+						EXT4_DIRENT_HASH(de),
+						EXT4_DIRENT_MINOR_HASH(de),
+						&de_name, &fstr);
 					de_name = fstr;
 					fstr.len = save_len;
 					if (err)